US 20090121136 A1
The electromagnetic radiation detector comprises at least one membrane suspended above a substrate by at least one nanowire. The nanowire forms a thermoelement comprising an electrically conducting core and external layer, respectively doped of different types and insulated from one another by an electrical insulation layer. When the substrate and membrane are at different temperatures, the nanowire constitutes a thermometer providing measurement signals, by Seebeck effect, representative of heating of the membrane.
1. An electromagnetic radiation detector comprising at least one radiation absorption membrane able to absorb electromagnetic radiation, to transform the radiation into heat and to transmit this heat to a thermometer, said membrane being suspended above a substrate by at least one nanowire substantially perpendicular to the substrate, and a detector, wherein the thermometer is formed by a thermoelement in contact with the membrane and the substrate and comprising at least said nanowire, each nanowire comprising a core and an external layer electrically insulated from one another at their periphery by an electrical insulation layer, the core and the external layer being electrically connected to the end of the nanowire, this end being in thermal contact with the membrane.
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11. A method for producing a detector according to
formation of electrical connection pads at the level of the substrate,
deposition of a droplet of catalyst on each connection pad,
growth of the nanowires at the location of the droplet of catalyst,
deposition of an electrical insulation layer on the nanowires and substrate,
deposition of an electrically conducting external layer on the nanowires and substrate,
deposition of a polymer resin embedding the nanowires,
planarization of the structure by chemical mechanical polishing,
formation of an electrical connection element between the core and the external layer and deposition and patterning of each absorption membrane,
removal of the polymer resin by chemical etching.
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The invention relates to an electromagnetic radiation detector comprising at least one radiation absorption membrane able to absorb electromagnetic radiation, to transform the radiation into heat and to transmit this heat to a thermometer, said membrane being suspended above a substrate by at least one nanowire substantially perpendicular to the substrate.
Electromagnetic radiation detectors comprises a sensitive element able to be heated by electromagnetic radiation. The temperature increase of the sensitive element gives rise to electrical charges occurring by pyroelectric effect. Operation of this type of detector with good performance requires three main conditions to be met as far as the sensitive element is concerned: a low calorific mass, a good thermal insulation of the sensitive element from its support, and a large sensitivity of the heat rise conversion effect into an electrical signal. This type of detector conventionally comprises a sensitive element absorbing electromagnetic radiation, suspended above a support substrate. The absorption means are fixed to the substrate by means of an anchoring point.
As illustrated in
The document EP-A-1653205 describes a bolometric detector wherein pillars 3 can be made from nanowires or nanotubes able to have a diameter of about 10 Å.
The article “Fabrication and characterization of a nanowire/Polymer-Based Nanocomposite for a Prototype Thermoelectric Device”, published in “Journal of microelectromechanical systems” Vol. 13 No. 3, June 2004, p. 505-513, describes the use of an array of silicon nanowires to form a thermoelement. As illustrated in
It is one object of the invention to provide an electromagnetic radiation detector, in particular of bolometer type, that presents good performances and is easy to fabricate.
This object is achieved by the fact that the thermometer is formed by a thermoelement in contact with the membrane and the substrate and comprising at least said nanowire, each nanowire comprising a core and an external layer electrically insulated from one another at their periphery by an electrical insulation layer, the core and the external layer being electrically connected to the end of the nanowire, this end being in thermal contact with the membrane.
It is a further object of the invention to provide a method for producing a detector successively comprising:
Other advantages and features will become more clearly apparent from the following description of particular embodiments of the invention given for non-restrictive example purposes only and represented in the accompanying drawings, in which:
The invention uses a nanowire-based thermoelement of the type described in U.S. patent application Ser. No. 11/826,293, claiming priority from French Patent application n° 0606617 filed on Jul. 20, 2006, to constitute the thermometer of the detector. As illustrated in
According to the embodiment illustrated in
The absorption membrane heats due to the effect of the electromagnetic radiation. The nanowire is in contact, at its base, with substrate 2 heated to a temperature T1, and at its top end with membrane 1 heated to a temperature T2 by the electromagnetic radiation. The nanowire is therefore subjected to a temperature gradient T1-T2 which generates a current in the nanowire by Seebeck effect. The generated current can thereby be measured by measuring means (not shown) integrated in the substrate, and the intensity of the corresponding electromagnetic radiation be deduced therefrom.
In an alternative embodiment, in the case where the membrane is not sufficiently electrically conducting, electrical connection between core 8 and external layer 9 at the top end of nanowire is achieved by deposition of a metallic layer (not shown) supporting the layer absorbing the electromagnetic radiation. This metallic layer can be made from nickel, titanium, chromium, copper, platinum or any other electrically conducting metal. Absorption membrane 1 or absorption layer is made from a material absorbing the electromagnetic radiation considered. For example in the infrared band, nickel, titanium, chromium, etc. can be used. Absorption membrane 1 can be made from the same material as that used for connection between core 8 and external layer 9 of the nanowire.
According to another embodiment illustrated in
A method for producing a detector comprising several elementary membranes each supported by a central pillar formed by a nanowire and arranged in the form of a bar or a matrix is illustrated in
First of all (
In an alternative embodiment, an electrical connection element (not shown) is produced at the end of each nanowire by deposition of an electrically conducting material, after planarization by chemical mechanical polishing, to electrically connect core 8 and external layer 9 of each nanowire. Then a layer is deposited and patterned to achieve elementary absorption membranes 1.
In the particular embodiment illustrated in
In the particular embodiment illustrated in
Each elementary absorption membrane 1 preferably has a thickness of 50 nm but which can be comprised between 10 and 300 nm. The shape of the membrane will preferably be square, but could also be circular, triangular or polygonal. The typical size of membrane 1 is 10×10 μm, but it can be comprised between 0.1 and 100 μm depending on the wavelength of the electromagnetic radiation to be measured and the required resolution.
The distance between the nanowires can be adjusted to the required value by depositing droplets of catalyst 11 on substrate 2 at specific locations. The distance between two nanowires is about 10,000 nm but can be comprised between 200 and 50,000 nm.
Each nanowire has for example a length of 2.5 microns. Its core 8 is made from conducting material, preferably doped silicon of a first type (for example p-doped), with a diameter of 15 nm. Electrical insulation layer 10, preferably made of SiO2, has for example a thickness of 2 nm. External layer 9 of each nanowire is made of conducting material, preferably silicon of a second type (n-doped) or from metal, with a thickness of 3 nm. The total diameter of the nanowire can be comprised between 10 and 100 nm.
Assuming that the thermal resistance of the nanowire is equivalent to that of a bulk nanowire of the same diameter, and that Fourier's law which assumes a diffusive behaviour of the phonons remains applicable, such a nanowire with a diameter of 25 nm has a thermal conductivity k of about 9.5 W/m.K, i.e. a thermal resistance Rth=L/kS.
L being the length of the nanowire and S the cross section of the heat flux passage within the nanowire, about 540 MK/W. This thermal resistance is much higher than that obtained with suspension arms according to the prior art, which is globally about 50 to 100 MK/W.
In practice, this thermal resistance may be higher. Electrical insulation layer 10, which accounts for about 20% of the heat flux passage, does in fact have a lower thermal conductivity, about 1 W/m.K, than the thermal conductivity involved above. Furthermore, the annular configuration provides an additional phonon diffusion effect at the interfaces. Doped silicon core 8 can further be produced by axial stacking of heterojunctions whose thermal conductivity is reduced by a factor 2 to 3. This type of heterojunction nanowire is for example described in U.S. Pat. No. 6,996,147. Increasing the length of the nanowire would result in a proportional increase of the thermal resistance, but the price to pay would be a cavity mismatch. For example, for a nanowire with a length of 10 microns, the thermal resistance would be multiplied by 4 and would reach a value of 2140 MK/W.
Depending on the radiation wavelength involved, the detector can work both in the close infrared (wavelengths from 0.7 to 5 microns) or medium infrared (from 5 to 30 microns), in the visible (wavelengths comprised between 400 and 700 nm), as well as in the ultraviolet and below (wavelengths comprised between 10 and 400 nm).
To improve the thermal insulation of the membrane, the whole detector can be placed in a vacuum or in a gas at very low pressure, behind a window transparent to the considered radiation.
The detector can also comprise cooling means in order to reduce the thermal noise. The substrate can also be kept at a set temperature by Peltier effect elements to increase the precision and reproducibility of the detector.
The use of a nanowire-based thermoelement as thermometer of the detector enables the electromagnetic radiation to be measured by means of the Seebeck effect. On account of the very low weight of the nanowires at their end, the response time of the thermoelement is reduced and the detector is more sensitive. The small diameter and the relatively large height of the nanowires further enables an excellent thermal insulation to be achieved between membrane 1 and substrate 2. Another advantage of such a detector stems from the fact that the detector itself generates an electric voltage. The electric consumption of the radiation detector is therefore reduced and only the electronic data analysis circuit has to be supplied with power, which is particularly advantageous for systems that have to be autonomous.